Purpose:
Normal visual experience is dependent upon accommodation and vergence responses providing adequate retinal image quality and correspondence. Image correspondence requires motor fusion, which must overcome any dissociated heterophoria. Infants and young children are typically hyperopic and do not wear refractive correction. We wished to test the hypothesis that they would exhibit esophoria at a near viewing distance as a result of their accommodative convergence, and determine the range of their motor fusion responses around this point.

Methods:
Eccentric photorefraction (PR) and Purkinje image eye tracking (MCS PowerRefractor) were used to record accommodation and eye alignment in adults and children less than 5 years of age. Participants viewed a naturalistic target at 80 cms. In the first condition, they viewed the target monocularly for 45 secs followed by a binocular period for 20 sec. Dissociated heterophoria was derived from the difference between these two alignments. In the second condition, motor fusion capacity was measured objectively by introducing base-in (BI) and base-out (BO) prisms (2-40pd). Adults also reported their subjective experience of diplopia.

Conclusions:
Despite typical levels of hyperopia, infants and children routinely exhibited exophoria for an 80cm target and demonstrated substantial fusional reserves to overcome this phoria. In typically developing children, these data suggest motor performance that is protective against refractive strabismus before emmetropization is complete.